You can get -15V by swapping leads, but NOT IN THE SAME CIRCUIT. The way to get -15 and +15 is to connect two 15V batteries in series, use the negative of the lower battery as -15V, the center as ground and the + of the upper 15V battery as +15V

If you need both +15V and -15V from AC, usually a center tap transformer is used, with center tap as ground.

If you need -15VDC from +15VDC (±15VDC at the same time), you can get a DC-DC converter, which is a switching power supply. There are many pre-built supplies out there.